This study is a case-control study investigating the causes of childhood leukemia in Northern California. The overall purpose of this epidemiologic study is to find specific genetic or environmental factors that may increase the risk of leukemia in children. The study is being conducted by Patricia Buffler, PhD at the School of Public Health - University of California Berkeley in collaboration with the California Department of Health Services and 16 hospitals located throughout the state of California. The study began in 1995 and will continue to 2014.